I am building a app with many different parts which access remote api calls (both my own, and others). There are many errors that might happen, and to exacerbate the problem, different libraries handle these errors differently.
Essentially, i would like to use the same error handling blocks for all these remote calls.
This is how i would do it with Ruby, but i am not that sure how to manipulate objective c in the same manner
//universal function to handle standard remote errors across errors
def universal_handling
begin
yield
rescue Exception => e
// handle different exceptions accordingly
// allow crash if unexpected exception
end
end
//how i would use the above block
universal_handling{ //any of my remote call here }
So, i have 2 questions (sample code very much appreciated)
How would I write the equivalent code in Objective-C? It is critical that I can use the same handling block throughout the app
In iOS dev, is this good practice?
Thanks for any help rendered! Error handling can be a major pain in the ass, so i do want to get this right early on =)
Notes:
Blocks are perfectly fine. I am not intending to support < 4.2 versions.
I read most of the articles out there, but none answers how you can use blocks to write "wrappers" for a specific set of calls.
You can do something very similar with blocks:
typedef void(^WrappableBlock)(void);
^(WrappableBlock block) {
#try {
block();
}
#catch(...)
}
//handle exception
}
}
However, it's very important to realize that the Cocoa (and CocoaTouch) libraries are not exception-safe. Throwing an exception through Cocoa frameworks will lead to all sorts of problems as the frameworks will not properly handle or clean up from the exceptions, leaving your application in a possibly inconsistent state. The correct Cocoa-style is to use NSError and return flags to indicate error conditions. This is neither better nor worse than using exceptions, just a different philosophy.
To do something similar to your universal_handling with NSError is not quite so straight forward because it will require that anything you call comply with the NSError pattern. That said:
typedef BOOL(^WrappableBlock)(NSError**);
^(WrappableBlock block, NSError **err) {
BOOL success = block(err);
if(!success) {
// handle error
}
return success;
}
would wrap any method that takes just an NSError** and returns a BOOL to indicate the presence of an error. Obviously the utility of this wrapper is limited as you'll have to wrap any interesting method in an other block to handle any other parameters. Of course, since it's the NSError** pattern, you can always just handle the errors when/where you want and pass NULL as the NSError** parameter where you don't care (ignoring the return value).
One final note: if you are using libraries that may throw exceptions, you must catch those exceptions within the scope of the library call and handle them. Do not let the exceptions propagate as they might then propagate through Cocoa framework code. Thus, the utility of the universal_handling block you propose is limited.
I would suggest you this nice cocoawithlove article that explains how to handle unhandled exceptions in Objective-C and also how to recover from an exception instead of "just" crashing. That could be a good alternative to what you are looking for.
I'm afraid that Objective-C not being as dynamic as ruby, that is a bit harder to do.
Would love to see a good block based implementation too, if that's possible, even If I'm not using them yet, because of previous version compatibility.
Related
The biggest misunderstanding for me in Swift is the throws keyword. Consider the following piece of code:
func myUsefulFunction() throws
We cannot really understand what kind of error it will throw. The only thing we know is that it might throw some error. The only way to understand what the error might be is by looking at the documentation or checking the error at runtime.
But isn't this against Swift's nature? Swift has powerful generics and a type system to make the code expressive, yet it feels as if throws is exactly opposite because you cannot get anything about the error from looking at the function signature.
Why is that so? Or have I missed something important and mistook the concept?
I was an early proponent of typed errors in Swift. This is how the Swift team convinced me I was wrong.
Strongly typed errors are fragile in ways that can lead to poor API evolution. If the API promises to throw only one of precisely 3 errors, then when a fourth error condition arises in a later release, I have a choice: I bury it somehow in the existing 3, or I force every caller to rewrite their error handling code to deal with it. Since it wasn't in the original 3, it probably isn't a very common condition, and this puts strong pressure on APIs not to expand their list of errors, particularly once a framework has extensive use over a long time (think: Foundation).
Of course with open enums, we can avoid that, but an open enum achieves none of the goals of a strongly typed error. It is basically an untyped error again because you still need a "default."
You might still say "at least I know where the error comes from with an open enum," but this tends to make things worse. Say I have a logging system and it tries to write and gets an IO error. What should it return? Swift doesn't have algebraic data types (I can't say () -> IOError | LoggingError), so I'd probably have to wrap IOError into LoggingError.IO(IOError) (which forces every layer to explicitly rewrap; you can't have rethrows very often). Even if it did have ADTs, do you really want IOError | MemoryError | LoggingError | UnexpectedError | ...? Once you have a few layers, I wind up with layer upon layer of wrapping of some underlying "root cause" that have to be painfully unwrapped to deal with.
And how are you going to deal with it? In the overwhelming majority of cases, what do catch blocks look like?
} catch {
logError(error)
return
}
It is extremely uncommon for Cocoa programs (i.e. "apps") to dig deeply into the exact root cause of the error and perform different operations based on each precise case. There might be one or two cases that have a recovery, and the rest are things you couldn't do anything about anyway. (This is a common issue in Java with checked exception that aren't just Exception; it's not like no one has gone down this path before. I like Yegor Bugayenko's arguments for checked exceptions in Java which basically argues as his preferred Java practice exactly the Swift solution.)
This is not to say that there aren't cases where strongly typed errors would be extremely useful. But there are two answers to this: first, you're free to implement strongly typed errors on your own with an enum and get pretty good compiler enforcement. Not perfect (you still need a default catch outside the switch statement, but not inside), but pretty good if you follow some conventions on your own.
Second, if this use case turns out to be important (and it might), it is not difficult to add strongly typed errors later for those cases without breaking the common cases that want fairly generic error handling. They would just add syntax:
func something() throws MyError { }
And callers would have to treat that as a strong type.
Last of all, for strongly typed errors to be of much use, Foundation would need to throw them since it is the largest producer of errors in the system. (How often do you really create an NSError from scratch compared to deal with one generated by Foundation?) That would be a massive overhaul of Foundation and very hard to keep compatible with existing code and ObjC. So typed errors would need to be absolutely fantastic at solving very common Cocoa problems to be worth considering as the default behavior. It couldn't be just a little nicer (let alone have the problems described above).
So none of this is to say that untyped errors are the 100% perfect solution to error handling in all cases. But these arguments convinced me that it was the right way to go in Swift today.
The choice is a deliberate design decision.
They did not want the situation where you don't need to declare exception throwing as in Objective-C, C++ and C# because that makes callers have to either assume all functions throw exceptions and include boilerplate to handle exceptions that might not happen, or to just ignore the possibility of exceptions. Neither of these are ideal and the second makes exceptions unusable except for the case when you want to terminate the program because you can't guarantee that every function in the call stack has correctly deallocated resources when the stack is unwound.
The other extreme is the idea you have advocated and that each type of exception thrown can be declared. Unfortunately, people seem to object to the consequence of this which is that you have large numbers of catch blocks so you can handle each type of exception. So, for instance, in Java, they will throw Exception reducing the situation to the same as we have in Swift or worse, they use unchecked exceptions so you can ignore the problem altogether. The GSON library is an example of the latter approach.
We chose to use unchecked exceptions to indicate a parsing failure. This is primarily done because usually the client can not recover from bad input, and hence forcing them to catch a checked exception results in sloppy code in the catch() block.
https://github.com/google/gson/blob/master/GsonDesignDocument.md
That is an egregiously bad decision. "Hi, you can't be trusted to do your own error handling, so your application should crash instead".
Personally, I think Swift gets the balance about right. You have to handle errors, but you don't have to write reams of catch statements to do it. If they went any further, people would find ways to subvert the mechanism.
The full rationale for the design decision is at https://github.com/apple/swift/blob/master/docs/ErrorHandlingRationale.rst
EDIT
There seems to be some people having problems with some of the things I have said. So here is an explanation.
There are two broad categories of reasons why a program might throw an exception.
unexpected conditions in the environment external to the program such as an IO error on a file or malformed data. These are errors that the application can usually handle, for example by reporting the error to the user and allowing them to choose a different course of action.
Errors in programming such as null pointer or array bound errors. The proper way to fix these is for the programmer to make a code change.
The second type of error should not, in general be caught, because they indicate a false assumption about the environment that could mean the program's data is corrupt. There my be no way to continue safely, so you have to abort.
The first type of error usually can be recovered, but in order to recover safely, every stack frame has to be unwound correctly which means that the function corresponding to each stack frame must be aware that the functions it calls may throw an exception and take steps to ensure that everything gets cleaned up consistently if an exception is thrown, with, for example, a finally block or equivalent. If the compiler doesn't provide support for telling the programmer they have forgotten to plan for exceptions, the programmer won't always plan for exceptions and will write code that leaks resources or leaves data in an inconsistent state.
The reason why the gson attitude is so appalling is because they are saying you can't recover from a parse error (actually, worse, they are telling you that you lack the skills to recover from a parse error). That is a ridiculous thing to assert, people attempt to parse invalid JSON files all the time. Is it a good thing that my program crashes if somebody selects an XML file by mistake? No isn't. It should report the problem and ask them to select a different file.
And the gson thing was, by the way, just an example of why using unchecked exceptions for errors you can recover from is bad. If I do want to recover from somebody selecting an XML file, I need to catch Java runtime exceptions, but which ones? Well I could look in the Gson docs to find out, assuming they are correct and up to date. If they had gone with checked exceptions, the API would tell me which exceptions to expect and the compiler would tell me if I don't handle them.
I have a few doubts regarding exception handling in iPhone. Here are they:
Suppose I have a chain of methods which are being called one after the other, that is, method A calls method B, which in turn calls method C, which calls method D. Which is the best place to put my try-catch block (is it method A or B or C or D or all of them). Also, I need to display an alert to user that an exception has occured and then, I want to log this exception to my server. So, if I am writing my try - catch block in all of this methods and if an exception occurs in method D; then I think the alert will be displayed 4 times and the web service for logging will also be called 4 times (till control reaches to catch block of method A). So, should I just use #throw; in catch block of method B, C and D and write my logic in catch block of method A (top level method) or should I avoid writing try - catch at all in methods B,C and D.
I need some sort of error code from the exception (because my web service needs parameters error-code and description). Is it possible to convert an exception to error or will I need to hard-code this code?
I had read somewhere about NSSetUncaughtExceptionHandler. And I think, if I can set this handler (in appDidFinishLaunching method of app delegate) and in the handler method, if I show to user some alert and call the web service; then I need not write try - catch block in each of my methods, in each of my classes. Am I right??
If an exception has occured and I have written either a try - catch block or NSSetUncaughtExceptionHandler, then will my app continue running or it will not respond to any of the user events. (I am sure it will handle the crash. What I want to know is whether it will hang)
Someone please enlighten me on this EXCEPTION TOPIC.
0) Avoid exceptions in Cocoa. They are generally non-recoverable. You might catch them for your own error reporting, but it's generally unsafe to assume you can recover from them.
1) If you need to catch, catch it immediately. Don't write your own throws -- instead, convert it to something like an NSError and pass that around. NSError can contain all the information you need to display or send an error code as well as a localized message.
2) You cannot convert an NSException into an NSError (directly) because an NSException does not have all the properties an NSError has - it is a different data representation. For one, an error code is not available. Two, the description is not localized. The best you can do is to create an error code and domain, then use the properties you need from the NSException and store that in an NSError. This could look something like the following:
// error checking omitted
extern NSString* const MONExceptionHandlerDomain;
extern const int MONNSExceptionEncounteredErrorCode;
NSError * NewNSErrorFromException(NSException * exc) {
NSMutableDictionary * info = [NSMutableDictionary dictionary];
[info setValue:exc.name forKey:#"MONExceptionName"];
[info setValue:exc.reason forKey:#"MONExceptionReason"];
[info setValue:exc.callStackReturnAddresses forKey:#"MONExceptionCallStackReturnAddresses"];
[info setValue:exc.callStackSymbols forKey:#"MONExceptionCallStackSymbols"];
[info setValue:exc.userInfo forKey:#"MONExceptionUserInfo"];
return [[NSError alloc] initWithDomain:MONExceptionHandlerDomain code:MONNSExceptionEncounteredErrorCode userInfo:info];
}
#catch (NSException * exc) {
NSError * err = NewNSErrorFromException(exc);
...
}
If the APIs you use throw exceptions you are expected to catch and recover from (e.g. not truly exceptional cases), then yes, you could catch and attempt to continue. Unfortunately, anybody that writes exceptions in Cocoa with the intent that you will catch them probably does not understand the issues well enough to implement a solid unwind implementation (e.g. even if it produces leaks, it's not solid).
3) That's really not the time or place to display an alert. If you install a top level exception handler (via NSSetUncaughtExceptionHandler) - You should simply log a message -- then the exception handler will abort. Your app is in an unstable state -- continuing is worse than aborting. You may want to send these custom messages home, it's best to do so at the next launch of your app.
4) In the majority of cases, your app is in an unstable state and you should not continue. But, to actually answer it for those corner cases: "Yes, you can recover and continue when you catch, but you should only attempt to recover and continue when the throwing API states that recovery is supported. If the problem is beyond your control, and the problem is not exceptional (e.g. file not found), and the vendor really expects you to continue, then I would have to assume that they expect you to continue, even though it really isn't (100% safe).". Do not attempt to recover/continue from within your top level exception handler (the program will abort after it returns). If you want to be very fancy and present that immediately on OSX, another process would be best. If you are calling through a pure C++ interface, then the unwinding is well defined, and the need to catch is necessary - do continue if it is recoverable. Exceptions in C++ can be recoverable and well defined - they are also used quite extensively (including less than exceptional conditions).
(IMO...) Exceptions in ObjC should not have been introduced, and any method that throws from system or third party libraries should be deprecated. They don't unwind well, or in a well defined manner. As well, unwinding flows against normal Cocoa program flow. That means that touching any objc object's memory/relations that was in mutation at the time of the throw and which lies between the throw and the catch is as good as undefined behaviour. Problem is - you have no idea what that memory is (in most cases, and within reasonable maintenance time). C++ exceptions are well defined, and they unwind correctly (e.g. destructors are called) - but trying to continue in an ObjC context is ignoring any consequences of undefined behavior. IMO, they should only exist for ObjC++ (because C++ requires them).
In an ideal world, your ObjC programs and the libraries you use would not use exceptions (at all). Since you use libraries that do throw (including Cocoa), install a top level exception handler only if you need some special information about the error. Where the API mandates that you could expect an exception thrown due to circumstances beyond your control and you are expected to recover, then write a catch but immediately convert that logic to normal program flow (e.g. NSError) - you never need to write your own throw. -[NSArray objectAtIndex: and "object does not respond to selector" are examples of programmer errors - they should not be caught, but the program should be corrected.
One of my friends asked me not to use NSException in iPhone Apps. The reason he gave was "performance bottleneck". But I am not convinced of it.
Can someone confirm me that we should restrain from using NSException in iPhone App? If you have best practices for using NSException, please provide that too.
UPDATE:
This link requests us to use Exception handling at the app level. Have someone ever done it? Please give the advantages of it and any other performance hitches it could create.
In short:
Do not use exceptions to indicate anything but unrecoverable errors
It is only appropriate to use #try/#catch to deal with unrecoverable errors. It is never appropriate to use #throw/#try/#catch to do control-flow like operations on iOS or Mac OS X. Even then, consider carefully whether you are better off using an exception to indicate an unrecoverable error or simply crashing (call abort()); a crash often leaves behind significantly more evidence.
For example, it would not be appropriate to use for catching out-of-bounds exceptions unless your goal is to catch them and somehow report the error, then -- typically -- crash or, at the least, warn the user that your app is in an inconsistent state and may lose data.
Behavior of any exception thrown through system framework code is undefined.
Can you explain the "Behavior of any
exception thrown through system
framework code is undefined." in
detail?
Sure.
The system frameworks use a design where any exception is considered to be a fatal, non-recoverable, error; a programmer error, for all intents and purposes. There are a very limited number of exceptions (heh) to this rule.
Thus, in their implementation, the system frameworks will not ensure that everything is necessarily properly cleaned up if an exception is tossed that passes through system framework code. Sine the exception is, by definition, unrecoverable, why pay the cost of cleanup?
Consider this call stack:
your-code-1()
system-code()
your-code-2()
I.e. code where your code calls into system code which calls into more of your code (a very common pattern, though the call stacks are obviously significantly deeper).
If your-code-2 throws an exception, that the exceptions passes over system-code means the behavior is undefined; system-code may or may not leave your application in an undefined, potentially crashy or data-lossy, state.
Or, more strongly: You can't throw an exception in your-code-2 with the expectation that you can catch and handle it in your-code-1.
I've used exception handling for my fairly intensive audio app with no problems whatsoever. After much reading and a bit of benchmarking and disassembly analysis, I've came to the controversial conclusion that there is no real reason not to use them (intelligently) and plenty of reason not to (NSError pointer-pointers, endless conditionals...yuck!). Most of what people are saying on the forums is just repeating the Apple Docs.
I go into quite a bit of detail in this blog post but I'll outline my findings here:
Myth 1: #try/#catch/#finally is too expensive (in terms of CPU)
On my iPhone 4, throwing and catching 1 million exceptions takes about 8.5 seconds. This equates to about only 8.5 microseconds for each. Expensive in your realtime CoreAudio thread? Maybe a bit (but you'd never throw exceptions there would you??), but an 8.5μs delay in the UIAlert telling the user there was a problem opening their file, is it going to be noticed?
Myth 2: #try Blocks Have a Cost On 32bit iOS
The Apple docs speak of "zero-cost #try blocks on 64bit" and state that 32 bit incurs a cost. A little benchmarking and disassembly analysis seems to indicate that there is zero-cost #try blocks on 32bit iOS (ARM processor) as well. Did Apple mean to say 32bit Intel?
Myth 3: It Matters that Exceptions Thrown Through Cocoa Frameworks are Undefined
Yes, they are "undefined", but what are you doing throwing through the Apple framework anyway? Of course Apple doesn't handle them for you. The whole point of implementing exception handling for recoverable errors is to handle them locally - just not every-single-line "locally".
One edge case here is with methods like NSObject:performSelectorOnMainThread:waitUntilDone:. If the later parameter is YES, this acts like a synchronous function in which case you might be forgiven for expecting the exception to bubble up to your calling scope. For example:
/////////////////////////////////////////////////////////////////////////
#pragma mark - l5CCThread
/////////////////////////////////////////////////////////////////////////
#interface l5CCThread : NSThread #end
#implementation l5CCThread
- (void)main
{
#try {
[self performSelectorOnMainThread:#selector(_throwsAnException) withObject:nil waitUntilDone:YES];
} #catch (NSException *e) {
NSLog(#"Exception caught!");
}
}
- (void)_throwsAnException { #throw [NSException exceptionWithName:#"Exception" reason:#"" userInfo:nil]; }
#end
/////////////////////////////////////////////////////////////////////////
#pragma mark - l5CCAppDelegate
/////////////////////////////////////////////////////////////////////////
#implementation l5CCAppDelegate
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
l5CCThread *thd = [[l5CCThread alloc] init];
[thd start];
return YES;
}
// ...
In this case the exception would pass "through the cocoa framework" (the main thread's run loop) missing your catch and crashing. You can easily work around this using GCD's dispatch_synch and putting in it's block argument the method call plus any exception handling.
Why Use NSException's over NSError's
Anyone who's ever done work in one of the older C-based frameworks like Core Audio knows what a chore it is checking, handling and reporting errors. The main benefit #try/#catch and NSExceptions provides is to make your code cleaner and easier to maintain.
Say you have 5 lines of code which work on a file. Each might throw one of, say, 3 different errors (eg out of disk space, read error, etc.). Instead of wrapping each line in a conditional which checks for a NO return value and then outsources the NSError pointer investigation to another ObjC method (or worse, uses a #define macro!), you wrap all 5 lines in a single #try and handle each error right there. Think of the lines you'll save!
By creating NSException subclasses, you can also easily centralise error messages, and avoid having your code littered with them. You can also easily distinguish your app's "non-fatal" exceptions from fatal programmer errors (like NSAssert). You can also avoid the need for the "name" constant (the subclass's name, is the "name").
Examples of all this and more details about the benchmarks and disassembly are on this blog post...
Exceptions plus try/catch/finally is the paradigm used by pretty much every other major language (C++, Java, PHP, Ruby, Python). Maybe its time to drop the paranoia and embrace it as well...at least in iOS.
Generally ask yourself if you're trying to signal an error, or do you actually have an exceptional condition? If the former, then it's a very bad idea regardless of any performance issue, real or perceived. If the latter, it's most definitely the right thing to do.
Apologies if this question has already been answered somewhere else, but I could not find any decisive answer when searching on it:
I'm wondering when try-catch blocks are to be used in objective-c iPhone applications. Apple's "Introduction to the Objective-C Programming Language" state that exceptions are resource intensive and that one should "not use exceptions for general flow-control, or simply to signify errors." From reading a few related questions here I also gather that people are not often using this method in practice.
So I guess the question is: what are the situations when it's appropriate to use try-catch blocks when developing for iPhone/Objective-C and when should they absolutely NOT be used?
For example, I could use them to catch beyond bounds and other exceptions when working with objects in arrays. I have a method which performs are few tasks with objects that are passed on in a number of arrays. The method returns nil if an error has occurred and a try-catch block could help me catch exceptions. However, I could of course simply write little if-tests here and there to ensure that I, for instance, never try to access an index beyond an arrays bounds. What would you do in this situation?
Thanks a lot!
It is only appropriate to use #try/#catch to deal with unrecoverable errors. It is never appropriate to use #throw/#try/#catch to do control-flow like operations.
In particular, it would not be appropriate to use for catching out-of-bounds exceptions unless your goal is to catch them and somehow report the error, then -- typically -- crash or, at the least, warn the user that your app is in an inconsistent state and may lose data.
Behavior of any exception thrown through system framework code is undefined.
Your if-test to do bounds checking is a far more appropriate solution.
#bbum's answer is absolutely correct (and he would know the answer better than most). To elaborate a bit...
In Cocoa, you should generally avoid using exceptions (#try/#catch[/#finally]) for flow control. As you mention, exceptions carry an unusually large cost (compared to run-times such as JVM or the CLR optimized for exception use). Furthermore, most of the Cocoa frameworks are not exception safe. Thus, throwing an exception through Cocoa framework code is dangerous and will likely cause odd, difficult to diagnose, and catastrophic (think possible data loss) bugs in your app.
Instead of using exceptions, Cocoa code uses NSError to signal error conditions that are recoverable within the application. Exceptions are used to signal conditions from which your application cannot recover. Thus a UI component requesting an out-of-bounds position in a model array could be signaled with an error (presented to the user with a reason their request could not be completed) while attempting to access an out-of-bounds position given an index that you think should be valid signals an exceptional condition where you app is in an inconsistent state and should probably die as soon as possible before it can do more damage.
NSParameterAssert, for example signals with an NSException when an assertion fails.
So when should you use exceptions or #try/#catch? If you're using a C/C++ library that makes use of exceptions, you should catch those exceptions before they can get thrown back through Cocoa code. Similarly, if you are serious about consistency of state within your application, you should raise an exception as soon as you detect that your state is inconsistent (and unrecoverable).
Question:
What is considered to be "Best practice" - and why - of handling errors in a constructor?.
"Best Practice" can be a quote from Schwartz, or 50% of CPAN modules use it, etc...; but I'm happy with well reasoned opinion from anyone even if it explains why the common best practice is not really the best approach.
As far as my own view of the topic (informed by software development in Perl for many years), I have seen three main approaches to error handling in a perl module (listed from best to worst in my opinion):
Construct an object, set an invalid flag (usually "is_valid" method). Often coupled with setting error message via your class's error handling.
Pros:
Allows for standard (compared to other method calls) error handling as it allows to use $obj->errors() type calls after a bad constructor just like after any other method call.
Allows for additional info to be passed (e.g. >1 error, warnings, etc...)
Allows for lightweight "redo"/"fixme" functionality, In other words, if the object that is constructed is very heavy, with many complex attributes that are 100% always OK, and the only reason it is not valid is because someone entered an incorrect date, you can simply do "$obj->setDate()" instead of the overhead of re-executing entire constructor again. This pattern is not always needed, but can be enormously useful in the right design.
Cons: None that I'm aware of.
Return "undef".
Cons: Can not achieve any of the Pros of the first solution (per-object error messages outside of global variables and lightweight "fixme" capability for heavy objects).
Die inside the constructor. Outside of some very narrow edge cases, I personally consider this an awful choice for too many reasons to list on the margins of this question.
UPDATE: Just to be clear, I consider the (otherwise very worthy and a great design) solution of having very simple constructor that can't fail at all and a heavy initializer method where all the error checking occurs to be merely a subset of either case #1 (if initializer sets error flags) or case #3 (if initializer dies) for the purposes of this question. Obviously, choosing such a design, you automatically reject option #2.
It depends on how you want your constructors to behave.
The rest of this response goes into my personal observations, but as with most things Perl, Best Practices really boils down to "Here's one way to do it, which you can take or leave depending on your needs." Your preferences as you described them are totally valid and consistent, and nobody should tell you otherwise.
I actually prefer to die if construction fails, because we set it up so that the only types of errors that can occur during object construction really are big, obvious errors that should halt execution.
On the other hand, if you prefer that doesn't happen, I think I'd prefer 2 over 1, because it's just as easy to check for an undefined object as it is to check for some flag variable. This isn't C, so we don't have a strong typing constraint telling us that our constructor MUST return an object of this type. So returning undef, and checking for that to establish success or failure, is a great choice.
The 'overhead' of construction failure is a consideration in certain edge cases (where you can't quickly fail before incurring overhead), so for those you might prefer method 1. So again, it depends on what semantics you've defined for object construction. For example, I prefer to do heavyweight initialization outside of construction. As to standardization, I think that checking whether a constructor returns a defined object is as good a standard as checking a flag variable.
EDIT: In response to your edit about initializers rejecting case #2, I don't see why an initializer can't simply return a value that indicates success or failure rather than setting a flag variable. Actually, you may want to use both, depending on how much detail you want about the error that occurred. But it would be perfectly valid for an initializer to return true on success and undef on failure.
I prefer:
Do as little initialization as possible in the constructor.
croak with an informative message when something goes wrong.
Use appropriate initialization methods to provide per object error messages etc
In addition, returning undef (instead of croaking) is fine in case the users of the class may not care why exactly the failure occurred, only if they got a valid object or not.
I despise easy to forget is_valid methods or adding extra checks to ensure methods are not called when the internal state of the object is not well defined.
I say these from a very subjective perspective without making any statements about best practices.
I would recommend against #1 simply because it leads to more error handling code which will not be written. For example, if you just return false then this works fine.
my $obj = Class->new or die "Construction failed...";
But if you return an object which is invalid...
my $obj = Class->new;
die "Construction failed #{[ $obj->error_message ]}" if $obj->is_valid;
And as the quantity of error handling code increases the probability of it being written decreases. And its not linear. By increasing the complexity of your error handling system you actually decrease the amount of errors it will catch in practical use.
You also have to be careful that your invalid object in question dies when any method is called (aside from is_valid and error_message) leading to yet more code and opportunities for mistakes.
But I agree there is value in being able to get information about the failure, which makes returning false (just return not return undef) inferior. Traditionally this is done by calling a class method or global variable as in DBI.
my $dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
But it suffers from A) you still have to write error handling code and B) its only valid for the last operation.
The best thing to do, in general, is throw an exception with croak. Now in the normal case the user writes no special code, the error occurs at the point of the problem, and they get a good error message by default.
my $obj = Class->new;
Perl's traditional recommendations against throwing exceptions in library code as being impolite is outdated. Perl programmers are (finally) embracing exceptions. Rather than writing error handling code ever and over again, badly and often forgetting, exceptions DWIM. If you're not convinced just start using autodie (watch pjf's video about it) and you'll never go back.
Exceptions align Huffman encoding with actual use. The common case of expecting the constructor to just work and wanting an error if it doesn't is now the least code. The uncommon case of wanting to handle that error requires writing special code. And the special code is pretty small.
my $obj = eval { Class->new } or do { something else };
If you find yourself wrapping every call in an eval you are doing it wrong. Exceptions are called that because they are exceptional. If, as in your comment above, you want graceful error handling for the user's sake, then take advantage of the fact that errors bubble up the stack. For example, if you want to provide a nice user error page and also log the error you can do this:
eval {
run_the_main_web_code();
} or do {
log_the_error($#);
print_the_pretty_error_page;
};
You only need it in one place, at top of your call stack, rather than scattered everywhere. You can take advantage of this at smaller increments, for example...
my $users = eval { Users->search({ name => $name }) } or do {
...handle an error while finding a user...
};
There's two things going on. 1) Users->search always returns a true value, in this case an array ref. That makes the simple my $obj = eval { Class->method } or do work. That's optional. But more importantly 2) you only need to put special error handling around Users->search. All the methods called inside Users->search and all the methods they call... they just throw exceptions. And they're all caught at one point and handled the same. Handling the exception at the point which cares about it makes for much neater, compact and flexible error handling code.
You can pack more information into the exception by croaking with a string overloaded object rather than just a string.
my $obj = eval { Class->new }
or die "Construction failed: $# and there were #{[ $#->num_frobnitz ]} frobnitzes";
Exceptions:
Do the right thing without any thought by the caller
Require the least code for the most common case
Provide the most flexibility and information about the failure to the caller
Modules such as Try::Tiny fix most of the hanging issues surrounding using eval as an exception handler.
As for your use case where you might have a very expensive object and want to try and continue with it partially build... smells like YAGNI to me. Do you really need it? Or you have a bloated object design which is doing too much work too early. IF you do need it, you can put the information necessary to continue the construction in the exception object.
First the pompous general observations:
A constructor's job should be: Given valid construction parameters, return a valid object.
A constructor that does not construct a valid object cannot perform its job and is therefore a perfect candidate for exception generation.
Making sure the constructed object is valid is part of the constructor's job. Handing out a known-to-be-bad object and relying on the client to check that the object is valid is a surefire way to wind up with invalid objects that explode in remote places for non-obvious reasons.
Checking that all the correct arguments are in place before the constructor call is the client's job.
Exceptions provide a fine-grained way of propagating the particular error that occurred without needing to have a broken object in hand.
return undef; is always bad[1]
bIlujDI' yIchegh()Qo'; yIHegh()!
Now to the actual question, which I will construe to mean "what do you, darch, consider the best practice and why". First, I'll note that returning a false value on failure has a long Perl history (most of the core works that way, for example), and a lot of modules follow this convention. However, it turns out this convention produces inferior client code and newer modules are moving away from it.[2]
[The supporting argument and code samples for this turn out to be the more general case for exceptions that prompted the creation of autodie, and so I will resist the temptation to make that case here. Instead:]
Having to check for successful creation is actually more onerous than checking for an exception at an appropriate exception-handling level. The other solutions require the immediate client to do more work than it should have to just to obtain an object, work that is not required when the constructor fails by throwing an exception.[3] An exception is vastly more expressive than undef and equally expressive as passing back a broken object for purposes of documenting errors and annotating them at various levels in the call stack.
You can even get the partially-constructed object if you pass it back in the exception. I think this is a bad practice per my belief about what a constructor's contract with its clients ought to be, but the behavior is supported. Awkwardly.
So: A constructor that cannot create a valid object should throw an exception as early as possible. The exceptions a constructor can throw should be documented parts of its interface. Only the calling levels that can meaningfully act on the exception should even look for it; very often, the behavior of "if this construction fails, don't do anything" is exactly correct.
[1]: By which I mean, I am not aware of any use cases where return; is not strictly superior. If someone calls me on this I might have to actually open a question. So please don't. ;)
[2]: Per my extremely unscientific recollection of the module interfaces I've read in the last two years, subject to both selection and confirmation biases.
[3]: Note that throwing an exception does still require error-handling, as would the other proposed solutions. This does not mean wrapping every instantiation in an eval unless you actually want to do complex error-handling around every construction (and if you think you do, you're probably wrong). It means wrapping the call which is able to meaningfully act on the exception in an eval.